The metal-oxide-semiconductor field effect transistor (TMOSFET) is one of the control switches with low power requirement, high switch speed, and small volume; hence it is widely used in the analog or digital circuit design of the computers and communication apparatus for operating, treating or memorizing lots of data.
Please refer to FIGS. 1(a)-(g), which are schematic diagrams showing the conventional process for fabricating the TMOSFET. The process comprises the following steps of: (a) providing a substrate 11 and forming an epitaxy layer 12 and an oxide layer 13 thereon (as shown in FIG. 1(a)); (b) defining the oxide layer 13 and the epitaxy layer 12 with a photolithographic process and an etching procedure to form a trench structure 14 (as shown in Fig. (b)); (c) removing the oxide layer 13 by an etching procedure and forming a gate oxide layer 15 on the surface of the epitaxy layer 12 and the inner sidewalls of the trench structure 14 and then filling the trench structure 14 with the polysilicon layer 16 (as shown in FIG. 1(c)); (d) etching partial of the polysilicon layer 16 and the gate oxide layer 15 until the gate oxide layer 15 having a thickness of about 200 Å is left on the surface of the epitaxy layer 12 (as shown in FIG. 1(d)); and (e) performing a body implantation and a body drive-in procedure to form a body structure 121 in the epitaxy layer 12 (as shown in FIG. 1(e)).
The conventional body drive-in procedure is performed under a high temperature to diffuse the impurities to the deeper area of the epitaxy layer 12 for meeting the requirement of the electricity, by means of that ions naturally diffuse from high concentration area to low concentration area. Generally, the body drive-in procedure in step (e) is performed in the furnance with oxygen gas. Under the circumstances with oxygen and high temperature, the surface of the epitaxy layer 12 without the cover of the polysilicon layer 16 after the etching procedure in step (c) is easily being oxidized, which results in the formation of the silicon oxide 122 as shown in FIG. 1(e). Therefore the silicon oxide 122 on the surface of the body structure 121 has to be etched back to a thickness about 200 Å in step (f) (as shown in FIG. 1(f)). Then the source implantation and the source drive-in procedure are performed on the body structure 121 in step (g) for forming a source structure 123 as shown in FIG. 1(g). Finally, after the following procedures for depositing the dielectric layer and forming the conductive metal layer are performed, the basic structure of the TMOSFET is accomplished.
As described above, the drive-in procedure for forming the body structure 121 is performed under a high temperature with oxygen gas and thus the silicon oxide 122 is easily formed on the surface of the body structure 121. For avoiding the affection of the silicon oxide 122 to the quality of the source implantation in step (g), the silicon oxide 122 has to be etched back to a thickness about 200 Å before the source implantation and the source drive-in procedure are performed. However, the etch back step is an additional step in the TMOSFET fabricating procedure, which may also cause the silicon loss at the trench top corner 124 as shown in FIG. 1(e) and FIG. 1(f).
In addition, please refer to FIG. 1(f) again; the trench top corner profile of the trench top corner 124 on the body structure 121 is changed with the formation of the slanting sharp corner 1241 after the silicon oxide 122 is etched back. The slanting sharp corner 1241 may cause the abnormal electric leakage and the damage of the profile of the silicon MESA (Si MESA) 125 of the TMOSFET. Besides, the source structure 123 is usually formed by an isotropic implantation and a source driving-in procedure in step (g), therefore the formation of the slanting sharp corner 1241 at the trench top corner 124 in step (f) may also affect the source junction profile and the source junction depth of the source structure 123 indirectly. The slanting sharp corner 1231 of the source structure 123 will be formed if the source structure 123 has a large source junction depth (as shown in FIG. 1(g)). Moreover, the slanting sharp corner 1231 may cause the reduction of the channel 126 of the body structure 121, the drop of the voltage threshold (Vt), the increase of the Idss, and even the punching through of the TMOSFET.
Please refer to FIG. 2, which is the scanning electron microscope (SEM) image of the TMOSFET fabricated by the conventional process. As shown in FIG. 2, the silicon oxide 122 formed on the surface of the body structure 121 in the body drive-in procedure under a high temperature with oxygen gas is etched back, and the profile of the trench top corner 124 is changed with the formation of the slanting sharp corner 1241 (pointed by the arrow), which may also cause the change of the profile of the source structure in the follow-up source implantation and drive-in procedure indirectly. In addition, the conventional TMOSFET shows 0% of yield, 0V of voltage threshold, and the maximal Idss by the wafer accept test (WAT).
Therefore, it is required to develop a method for fabricating the TMOSFET to simplify the fabricating procedure and avoid the silicon loss at the trench top corner.